Chalcogenide glasses are made up of group IV-VI elements, group II-VI elements, or other combinations. The group VI chalcogenides include oxygen, sulfur, selenium, tellurium. Of considerable interest is their transmission throughout the infrared region and their non-linear refractive index. Further, such materials have high chemical stability. Common applications for these glasses include night vision systems and thermal imaging. Both applications exploit the relatively high transmissivity of the chalcogenide glass throughout the infrared region of light.
Anti-reflection (AR) is of major importance in a variety of systems, particularly in systems where multiple pieces of glass are used. Even a small reflection, if multiplied by a large number of glass surfaces, can result in significant transmission losses. By applying AR coatings, transmission can be maximized for enhanced efficiency and transmission quality. Another major factor to be considered is the refraction of the light when it passes through a piece of glass. When light passes through a medium it is bent, thereby traveling slower. This change in the speed which light encounters can cause reflection. In window glass, such as a in a car window, the light is only bent a small amount and, therefore, only a small reflection is produced. However, for chalcogenide glass, the light is bent more significantly. Accordingly, AR coatings are particularly important for chalcogenide glasses.
Increasing transmission results from a reduction in reflection. To provide an AR coating layer on a substrate to optimize transmission for one specific wavelength requires that the coating layer to have both a specific thickness and a specific refractive index. The simplest interference AR coating consists of a single quarter-wave thick layer of transparent material whose refractive index is the square root of the underlying substrate's refractive index (ncoating=(nsubstrate)1/2. This theoretically gives zero reflectance at the center wavelength and decreased reflectance for wavelengths in a broadband around the center.
Multi-layer AR coatings can be used to expand the properties of a single layer, among many layers. Using this arrangement, many layers of different thicknesses and refractive indices can be layered to allow for enhanced transmission for different wavelengths.
Current AR coating fabrication techniques are generally limited to two broad categories. The first involves the creation of a coating that has very high transmission for a certain wavelength of light. This technique is very useful for applications that use particular wavelengths of light, as in most lasers. The second type is termed broadband coatings. Some of the techniques used to create these functional coatings include multilayer coatings (MLC), sub-micron structures, and sol-gel derived AR coatings.
The sol-gel process is a versatile well-known low temperature solution process for making inorganic ceramic and glass materials. In general, the sol-gel process involves the transition of a system from a liquid “sol” (mostly colloidal) into a solid “gel” phase. Applying the sol-gel process, it is possible to fabricate ceramic or glass materials in a wide variety of forms including ultra-fine or spherical shaped powders, thin film coatings, ceramic fibers, micro porous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials.
The starting materials used in the preparation of the “sol” are usually alkoxides. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or a “sol”. Hydrolysis of an alkoxide liberates alcohol and results in polymerized chains of metal hydroxide. For example, silica gels can be formed by hydrolysis of tetraethoxysilicate (TEOS; an alkoxide having the formula Si(OC2H5)4) based on the formation of silicon oxide SiO2 and ethyl alcohol C2H5OH as noted below:Si(OC2H5)4+4H2O→Si(OH)4+4C2H5OH;Si(OH)4→SiO2+2H2O
Multilayer coatings are effective for transmission across a large range of wavelengths. The two major drawbacks of this technique are intrinsic to the properties of complicated coatings. The challenge becomes how to maintain coating thickness while minimizing defects. Further, multilayer coatings can be mechanically unsound when compared to single layers.
Finally, sol-gel derived AR coatings, being the easiest to apply, show the most potential in broadband applications. Unlike MLCs, single layer sol-gel coatings are very mechanically sound and are simple to create with minimal defects. Also, similarly to sub-micron structured AR techniques, sol-gel derived AR coatings use sub-micron particles to affect both the refractive index and the light scattering within the coating.
One of the most common coating materials used for antireflective properties is silica (SiO2). However, other material combinations can be used to achieve similar refractive index values while still using the mechanically strong sol-gel derived thin film. Examples of alternative materials that use a particular chalcogenide glass as a thin film are As2S3 or Ge45Se55. These materials have very low absorbance in the infrared region and combinations of these materials can be used to alter the refractive index through a wide range. Similar systems can be developed that include the use of many different coating materials to achieve the desired refractive gradient, such as an As2Se3/BaF2/air system, which depends on the densification of the film through heat treatments for mechanical stability. Although such a coating is generally feasible for obtaining the desired optical qualities, the strength of the film cannot be fully achieved without subsequent heat treatments to at least several hundred degrees C. to densify, reduce porosity, and create stronger linkage between constituent elements. However, such heat treatments can degrade optical properties of the films. What is needed is a new composition and low temperature process for forming broadband single layer AR coatings.